1. The hepatic extraction ratio of 14 C-labelled bile acids has been measured directly by hepatic vein catheterization in five patients without liver disease (glycocholic acid, three; cholic acid, two) and in 16 patients with histologically confirmed liver disease (glycocholic acid, seven; cholic acid, nine). 2. After intravenous administration of [ 14 C]-glycocholic acid by bolus injection (two control subjects) or constant infusion (one control subject), directly measured hepatic extraction ratio was 0.91, 0.84 and 0.88, greater than that for indocyanine green. The extraction ratio of [ 14 C]cholic acid in two subjects was 0.72 and 0.70, confirming a lower extraction of the unconjugated bile acid. 3. The hepatic extraction ratio of both bile acids was reduced in patients with chronic liver disease (range 0.07–0.69), although the extraction ratio of glycocholic acid remained normal in one patient with viral hepatitis. 4. Estimates of liver flow calculated from the extraction of [ 14 C]glycocholic acid, but not cholic acid, correlated with those calculated from indocyanine green kinetics, although numbers were small. 5. Measurement of the hepatic extraction of individual bile acids, not previously reported in man, allows a more accurate description of the enterohepatic circulation.